1. Field of the Invention
The present invention relates to a method of controlling a pulse width modulation or PWM inverter and an apparatus for carrying out the method. In particular, the invention concerns a method and apparatus for controlling a pulse width modulation of the PWM inverter in which a staircase wave is made use of as a modulating wave.
2. Description of the Prior Art
As one of the methods of controlling PWM inverters, there has heretofore been known a method in which a triangular wave is used as a carrier wave while a rectangular wave is used as the modulating wave, as is disclosed in the specification of U.S. Pat. No. 3,663,247, issued May 9, 1972 (in particular refer to FIG. 4f to FIG. 4k and associated descriptions). According to this prior art method, the triangular wave and the rectangular wave are compared with each other in respect of the voltage level. When the triangular wave is at a lower level than that of the rectangular wave, a firing pulse is produced. Otherwise, the firing pulse is not produced. The pulse width modulation of the firing pulse is effected by increasing or lowering the voltage level of the rectangular wave. This is a typical one of the hitherto known methods of controlling the PWM inverter.
The method of controlling the PWM inverter in which the rectangular wave is used as the modulating wave is certainly advantageous in that the modulating wave generator circuit can be implemented in a relatively simplified configuration. However, this method suffers a drawback that the output signal from the inverter is inflicted with numerous harmonics.
Another method of controlling the PWM inverter is also known from Japanese Laid-Open Patent Application No. 80526/1974 (in particular, FIG. 2), for example. According to this known method, a staircase wave is used as the modulating wave in place of the rectangular wave. The number of the stair steps of the staircase wave can be expressed by (N-1)/2, where N represents the number of the carrier waves appearing during a half period of the staircase wave.
The method in which the staircase wave is used as the modulating wave is advantageous in that the harmonic components are reduced. However, for an increased value of N, the number M of the stair steps has to be necessarily increased, which brings about the disadvantage that the modulating wave generator circuit becomes correspondingly complicated.